In recent years, fiber-optic chemical sensors have been developed to detect the presence and monitor the concentration of various analytes such as oxygen and carbon dioxide gas, as well as pH. Such sensors are based on the recognized phenomenon that the absorbance and, in some cases, the luminescence, phosphorescence, or fluorescence of certain indicator molecules are perturbed in the presence of specific analyte molecules. The perturbation of the light emission properties and/or absorbance profile of an indicator molecule can be detected by monitoring radiation that is absorbed, reflected, or emitted by it when illuminated in the presence of a specific analyte.
Fiber-optic probes that position an analyte sensitive indicator molecule in a light path optically monitor the effect of the analyte on the indicator molecule. Typically, for monitoring carbon dioxide or pH level, the optical fiber transmits electromagnetic radiation from a light source to the indicator molecule, and the level of absorbance as measured by the light reflected from the vicinity of the indicator molecule gives an indication of the gaseous analyte or hydrogen ion concentration. Alternatively, for monitoring other types of gases, such as O.sub.2, the optical fiber transmits electromagnetic radiation to the indicator molecule, exciting it to emit light, e.g., to phosphoresce. The duration of phosphorescence by the indicator molecule serves as an indication of the concentration of the gas in the surrounding fluid. These indicator molecules are typically disposed in a sealed chamber at the distal end of the optical fiber, with the chamber walls being permeable to the analyte, but not permeable to liquids.
One problem with known sensing systems of the type described is that the optical fiber and sealed chamber attached to the end of the probe are prone to physical damage. The optical fibers and attached sensors are delicate because they are situated as an external appendage located at the end of a catheter used to invasively insert the probe and extend distally beyond it. Any mishandling of the catheter can easily result in damage to the delicate chamber or optical fiber.
Another problem with the systems described above is that the probe can encourage the formation of blood clots, or thrombi. Prior art multifiber sensors provide interfiber crevices, which encourage thrombi formation, even in the presence of an anti-coagulant heparin solution.
A sensor disposed at the distal end of an optical fiber is sometimes subject to a phenomenon referred to as "wall effect" wherein the sensor impinges on the inner wall of an artery or vein and monitors the O.sub.2 concentration or other parameter at the vessel wall rather than measuring the parameter in the blood circulating through the vessel. A significant gradient can exist between the measured level of the parameter in free flowing blood at a position close to the center of the vessel and at the vessel wall. A sensor that is relatively small in diameter and is mounted at a distal-most end of an optical fiber is more likely to experience an error due to wall effect than one that comprises part of a multi-sensor bundle of larger overall diameter, because of the more limited surface area of the smaller sensor and its inherent propensity to lie closer to the vessel wall.